The knee joint comprises the interface between the distal end of the femur and the proximal end of the tibia. In a properly-functioning knee joint, medial and lateral condyles of the femur pivot smoothly along menisci attached to respective medial and lateral condyles of the tibia. When the knee joint is damaged, the natural bones and cartilage that form the joint may be unable to properly articulate, which can lead to joint pain and, in some cases, interfere with normal use of the joint.
In some situations, surgery is required to restore normal use of the joint and reduce pain. Depending upon the severity of the damage, the surgery may involve partially or completely replacing the joint with prosthetic components. During such knee replacement procedures, a surgeon resects damaged portions of the bone and cartilage, while attempting to leave healthy tissue intact. The surgeon then fits the healthy tissue with artificial prosthetic components designed to replicate the resected tissue and restore proper knee joint operation.
One knee replacement procedure—total knee arthroplasty (“TKA”)—involves the resection of some or all of each of the medial and lateral condyles of both the femur and tibia and the removal of the fibro-cartilage menisci located at the femorotibial interface. A prosthetic femoral component, typically made of titanium or other strong, surgical-grade metal, is fitted and secured to the distal end of the femur to replace the resected portion of the femur. Similarly, a prosthetic tibial component, the base of which is also typically made of titanium or other suitable metal, is fitted and secured to the proximal end of the tibia to replace the resected portion of the tibia.
In some situations, the patient's bone at the knee joint may have deteriorated to a point which requires TKA surgery, but one or more of the patient's cruciate ligaments (e.g., the anterior cruciate ligament (ACL) and/or posterior cruciate ligament (PCL) are in sufficient condition to provide adequate joint stability. Maintaining the native cruciate ligaments is often advantageous, as doing so is generally thought to aid in proprioception (the ability to sense where parts of the body are in relation to each other) and could make activities like climbing stairs feel more stable or natural. Preserving the cruciate ligaments can also promote more normal front to back knee motion, which can enhance the patient's ability to maintain pre-operative range of motion, particularly as it relates to deep flexion.
Each of the native cruciate ligaments connects to one of the femoral condyles and passes within the intercondylar region of the femur to connect the center-top portion of the tibia, called the tibial eminence. As such, preserving the cruciate ligaments requires that both the femoral and tibial implant components be configured to allow the cruciate ligaments to pass through the intercondylar area of the joint so that proper function and movement of the native ligaments is substantially preserved. Accordingly, the femoral implant component for cruciate retaining procedures is typically designed with features that replicate the form and function of the medial and lateral condyles of the femur. The medial and lateral components are separated by a deep intercondylar “notch” that allows for passage of cruciate ligaments through the notch. To ensure that the condyles do not interfere with any portion of a ligament during flexion and extension of the femur, some manufacturers design the implant to provide an intercondylar notch having a substantially symmetric “U”-shape having a width that generally corresponds to the widest dimension of the combination of the cruciate ligaments.
Furthermore, some prosthetic implant manufacturers create symmetrical implant components that can be easily scaled to accommodate patients with different bone sizes. Other manufacturers use designs that attempt to simplify the shape of the components using combinations of relatively basic lines and arcs. These designs are aimed at reducing costs that are generally associated with the design and manufacture of more complex, anatomically-accurate implant components. For example, the U-shaped design for the intercondylar notch described above reduces the manufacturing complexity that may be associated with more customized designs that attempt to replicate asymmetrical features of the native intercondylar area.
Although the approach for designing the intercondylar notch described above may ensure that the medial and lateral condyle components do not interfere with the cruciate ligaments, it may have several disadvantages. For example, because the combined width of the medial condyle, lateral condyle, and intercondylar notch is limited, an increase in the width of the intercondylar notch requires a corresponding decrease in the width of the medial and lateral condyles of the implant component. Reducing the width of these components may significantly reduce the strength of the femoral implant component.
Furthermore, although simplifying the design of the implant by using symmetrical components or basic “best fit” shapes may reduce manufacturing costs in some situations, such simplification may also lead to components that do not accurately replicate or fit the native anatomy of the joint. For example, certain features of an improperly-sized femoral implant may be larger than the surrounding bone and may impinge upon the surrounding soft tissue. Such imposition of the surrounding soft tissue by the implant, particularly during movement of the implant during flexion and extension of the joint, can cause significant discomfort and may potentially lead to permanent damage to the surrounding soft tissue.
The presently disclosed femoral implant for cruciate retaining knee arthroplasty procedures and methods for designing the same are directed to overcoming one or more of the problems set forth above and/or other problems in the art.